+1 on headspace reset. I just had a similar problem when I forgot to remove the .473" bolt head form a new 223 AI build. I set the headspace on the 308 BH before I caught it. Changing the bolt head to the proper .378" and the bolt would not close so I had to pull the rifle apart and reset the head space. Now all is well and the rifle is shooting between .3s and .6s with the first loads tested.